This invention relates to an improvement in apparatus for measuring the thickness of a thin metal coating on a substrate utilizing a coulometric technique.
Conventional coulometric measuring instruments utilize an electrolytic cell to electrolytically strip a thin metal coating on a substrate in making a measurement. The electrolytic cell includes a container for the electrolyte and a gasket having a central opening in fluid communication with the interior of the electrolyte container. In using this type of apparatus, the electrolyte container and the gasket are held against the plated substrate and the container filled with electrolyte. Sufficient force is applied to the container to prevent leakage of the electrolyte between the gasket and the workpiece. With this apparatus the electrolyte is held in intimate contact with the thin metal layer located within the central opening of the gasket. The gasket in conventional apparatus provides two functions--first, containment of the electrolyte and second, defining the area to be deplated. The electrolytic cell further includes a cathode within the interior of the container connected to a current source providing a known current. The workpiece is also connected to the current source, such that the workpiece is the anode of electrolytic cell, thus completing the electrical circuit.
Once the electrolytic cell and gasket are in position and filled with electrolyte, the current source is turned on and deplating of the plated layer within the area defined by the gasket commences. The potential across the electrolytic cell is measured on a timed basis. During deplating, this potential is approximately constant. However, once the metal layer is deplated, the potential abruptly changes to a different potential. The time from when the current source is turned on to when the potential so changes is indicative of the plating thickness. The time is converted to units of thickness by apparatus connected to the electrolytic cell.
For more accurate results, it is common to agitate the electrolyte to remove the bubbles forming on the metal layer being deplated and to prevent localized depletion of the electrolyte. If the bubbles are not removed or if the electrolyte becomes locally depleted, the time required to deplate the metal layer would be increased, thus affecting the accuracy of the measurement. Bubbles prevent the electrolyte from being in intimate contact with the metal layer, and depletion of the electrolyte severely alters the deplating conditions.
In conventional coulometric measuring instruments, the gasket is generally formed of a resilient material to achieve a leakage-free seal around the edge of the gasket when the electrolyte container is brought into engagement with the gasket and pressed against the workpiece. A conventional electrolytic cell with a gasket is shown in FIG. 1. The gasket shown has a minimum dimension A required to achieve sufficient rigidity to consistently maintain the critical dimention B defining the diameter of the area to be deplated when the gasket is pressed against the workpiece. There is an annular region C adjacent the outer edge of the area being deplated which is relatively stagnant because it does not receive the same degree of agitation as the center of the area being deplated. Thus, in this region, the benefits of agitation are not partically realized. Furthermore, because of the relatively large dimension A required, there is a practical limitation on the smallest area capable of being deplated for purposes of making a thickness measurement. The reason for this is the inability to properly agitate the entire volume of the electrolyte adjacent to the area being deplated when this volume is small. It has been found desirable to maintain the ratio of A/B to be less than approximately 1.0 to assure proper agitation. In order to maintain the ratio A/B at less than 1.0, the minimum diameter dimension B has been limited in commercial units by the minimum thickness dimension A required to achieve sufficient mechanical rigidity of the gasket necessary to contain the electrolyte and at the same time, accurately define the area to be deplated.
Since the gasket in commercial units available today is resilient, to provide a proper seal between the gasket and the workpiece, it is critical, for accurate, repeatable measurements, for the electrolyte container to be pressed against the gasket with precisely the same force during a single measurement, and further, during subsequent measurements. The reason for this is that the area being deplated changes with the force applied. The variation in area bring deplated affects the time period required to deplate the metal layer and, therefore, the accuracy of the measurement.
Finally, it is not possible with commercially available units to measure workpieces having small radii of curvature because of the distortion of the area being defined by the resilient gasket when an attempt is made to prevent leakage of the electrolyte by increasing the pressure on the gasket against the workpiece.